1. Field of the Invention
The present invention relates generally to a method for reducing byproducts emissions from combustion reactions, and, more particularly, to a method for regulating flue gas pollutants in reactors and combustion furnaces.
2. Description of the Prior Art
The combustion of sulfur-containing carbonaceous compounds, especially coal, results in a combustion product gas containing unacceptably high levels of sulfur dioxide.
SO2 Reduction Methods
The reduction of sulfur dioxide (SO2) is of primary concern to the power and industrial boiler industries since acid rain is a product of gaseous SO2 release into the environment. To combat acid rain, federal regulations are increasingly more stringent and companies are increasingly more exposed to federal penalties for over emitting SO2.
Sulfur dioxide is a colorless gas that is moderately soluble in water and aqueous liquids. It is formed primarily during the combustion of sulfur-containing fuel or waste. Once released to the atmosphere, sulfur dioxide reacts slowly to form sulfuric acid (H2SO4), inorganic sulfate compounds, and organic sulfate compounds.
Air pollution control systems for sulfur dioxide removal are large and sophisticated, and rely on two main techniques for sulfur dioxide removal: absorption and adsorption. Both methods rely on neutralization of the absorbed sulfur dioxide to an inorganic salt by alkali to prevent the sulfur from being emitted into the environment. The alkali for the reaction most frequently used include: limestone—either calcitic or dolomitic; quick and hydrated lime—slurry or dry; and magnesium hydroxide—commercial and byproduct from Thiosorbic lime; and trona.
Absorption—Absorption processes use the solubility of sulfur dioxide in aqueous solutions to remove it from the gas stream. Once sulfur dioxide has dissolved in solution to form sulfurous acid (H2SO3), it reacts with oxidizers to form inorganic sulfites (SO3−) and sulfates (SO4−). This process prevents the dissolved sulfur dioxide from diffusing out of solution and being re-emitted. The solution is then processed to remove the sulfur
Limestone is the alkali most often used to react with the dissolved sulfur dioxide. Limestone slurry is sprayed into the sulfur dioxide-containing gas stream. The chemical reactions in the recirculating limestone slurry and reaction products must be carefully controlled in order to maintain the desired sulfur dioxide removal efficiency and to prevent operating problems. Wet scrubbers used for sulfur dioxide control usually operate at liquid pH levels between 5 to 9 to maintain high efficiency removal. Typical removal efficiencies of sulfur dioxide in wet scrubbers range from 80 to 95%.
Another type of absorption system is called a spray atomizer dry scrubber, which belongs to a group of scrubbers called spray-dryer-type dry scrubbers. In this case, an alkaline slurry is sprayed into the hot gas stream at a point upstream from the particulate control device. As the slurry droplets are evaporating, sulfur dioxide absorbs into the droplet and reacts with the dissolved and suspended alkaline material.
Large spray dryer chambers are used to ensure that all of the slurry droplets evaporate to dryness prior to going to a high efficiency particulate control system. The term “dry scrubber” refers to the condition of the dried particles approaching the particulate control system. Fabric filters or electrostatic precipitators are often used for high efficiency particulate control.
Spray-dryer-type absorption systems have efficiencies that are similar to those for wet-scrubber-type absorption systems. These generate a waste stream that is dry and, therefore, easier to handle than the sludge generated in a wet scrubber. However, the equipment used to atomize the alkaline slurry is complicated and can require considerably more maintenance than the wet scrubber systems. Spray-dryer-type absorption systems operate at higher gas temperatures than wet scrubbers do and are less effective for the removal of other pollutants in the gas stream such as condensable particulate matter.
The choice between a wet-scrubber absorption system and a spray-dryer absorption system depends primarily on site-specific costs. The options available for environmentally sound disposal of the waste products are also an important consideration in selecting the type of system for a specific application. Both types of systems are capable of providing high efficiency sulfur dioxide removal. Both types of systems are also very expensive to install, operate, and maintain.
Adsorption—Sulfur dioxide can also be collected by adsorption systems. In this type of control system, a dry alkaline powder is injected into the gas stream. Sulfur dioxide adsorbs to the surface of the alkaline particles and reacts to form compounds that can be precipitated out of the gas stream. Hydrated lime (calcium hydroxide) is the most commonly used alkali; however, a variety of alkalis can be used effectively. A dry-injection-type dry scrubber can be used on smaller systems as opposed to using the larger, more complicated spray-dryer-type dry scrubber. However, the dry injection system is slightly less efficient, and requires more alkali per unit of sulfur dioxide (or other acid gas) collected. Accordingly, the waste disposal requirements and costs are higher for adsorption systems than absorption systems.
In general, the prior art adsorption methods are more expensive because they require expensive equipment, including a bag house and electrostatic precipitator, are inefficient in the utilization of alkali and reduction of sulfur, and require extra maintenance because the injectors are prone to plugging.
Thus, prior art SO2 absorption and adsorption systems and methods have drawbacks that make them expensive and/or inefficient. A need exists, therefore, for a SO2-removal system and method that produces an easily-handled by-product, achieves greater than 70% sulfur removal from the flue gas with high sorbent utilization, and reduces the equipment requirements (and costs).
Furnace Sorbent Injection (FSI) to Reduce SOx
Other pollutants, such as SO3, Hg, HCl, NOx, and PM have also been removed from combustion effluent by furnace sorbent injection (FSI). However, the prior art methods for the removal of these pollutants are also relatively inefficient and expensive to perform.
ROFA
Rotating opposed-fired air (ROFA) utilizes the co-ordinated, reinforcing, tangential injection of high-velocity secondary air to produce turbulent mixing, resulting in a greater combustion efficiency for greater NOx reduction, such as taught in U.S. Pat. No. 5,809,910 issued Sep. 22, 1998 to Svendssen, which describes a ROFA system that provides for the asymmetrical injection of overfired air (OFA) in order to create a rotation and high turbulence in the furnace, thus more thoroughly mixing the secondary air and the combustion gases. ROFA has been applied to combustion furnaces solely for the reduction of NOx and SO3 in the prior art.
Overall, while the use of boosted over fire air is known in the art, its use in combination with fuel sorbent injection for efficient, cost-effective, and highly effective reduction of pollutants has not been taught or disclosed in the prior art. Thus, a need exists for systems and methods providing for the reduction of pollutants in flue gas concentration in a combustion process burning sulfurous fossil fuel and utilizing a high-turbulence over fired air system.